The invention concerns current sensors of the closed-loop type, having a compensation circuit with secondary coil for cancelling the magnetic flux induced in the magnetic circuit. Closed-loop current sensors are widely used in a large number of current and voltage measurement applications. In such sensors, the magnetic flux in the magnetic circuit is very low, since the compensation coil, also commonly called secondary coil, is driven in a feedback loop to cancel the magnetic field induced by the current to be measured (primary current), the resultant magnetic field being picked up by a magnetic field sensor which thus needs to have a high sensitivity for accurate and rapid response of the current sensor. Various magnetic field sensors including Hall effect detectors, pick-up coils, or fluxgate sensors, may be employed in so-called closed-loop current sensors. The magnetic field detector may be arranged in an air-gap of the magnetic circuit, or in a partial air-gap, or in proximity of an air-gap, or simply in proximity or around a branch of the magnetic circuit.
In order to reduce the influence of external magnetic fields, considering the sensitivity of certain magnetic field sensors, it is advantageous to position the magnetic field sensor within the magnetic core or alongside the inner periphery, as opposed to an outer side of the magnetic coil where external magnetic fields have a greater influence on the magnetic field sensor. A closed-loop current sensor with a sensitive magnetic field detector is for example described in German patent application DE 102005028572. In the latter publication, the current sensor has a magnetic core that is made of a uniform width strip of soft magnetic material (i.e. material with a high magnetic permeability) folded into two identical parts that are assembled together in mirror symmetry. The magnetic core has extensions forming a space therebetween within which a magnetic field fluxgate sensor is positioned. The space for the magnetic field detector is bounded by an outer pair and an inner pair of arms, the outer pair being supposedly in edge abutting relationship, and the inner pair of arms supposedly providing an air-gap. Discontinuities in the magnetic circuit, such as between the abutting outer arms of the magnetic circuit, have a parasitic air-gap effect that influences the measurement accuracy and response of the sensor. A slight variation in the degree of contact between the abutting edges, or the creation of a slight gap, may have an important effect on the magnetic flux lines and thus the measurement accuracy of the sensor. The sensor described in the aforementioned publication is thus very sensitive to assembly accuracy and repeatability. The magnetic core is made of folded strip material that has a low rigidity and requires careful handling during manufacture and assembly. Moreover, the magnetic fluxgate sensor positioned in the space between first and second pairs of arms is also somewhat exposed to external field influences from the open sides.
In EP2083277, certain of the drawbacks mentioned above are overcome, however there is a desire to further reduce manufacturing costs while increasing the performance and reliability of the transducer, which may include aspects such as measurement accuracy, large measurement operating range, reduced sensitivity to disturbing external magnetic fields, manufacturing repeatability, stability over time and usage, and robustness.